Field of the Invention
The present invention generally relates to a charged particle beam exposure method and system, such as an electron beam exposure method and system. More particularly, the present invention is concerned with a process for extracting appropriate block pattern exposure data used when a wafer exposure is performed by projecting an electron beam onto a transparent mask placed above a wafer.
A variable rectangle-shaped electron beam exposure process is widely used, in which a pattern is continuously drawn on a wafer by means of an electron beam in a "single stroke of the brush" manner.
Recently, an electron beam exposure system performing a block exposure process has been developed in order to produce highly-integrated circuits, such as large scale integrated circuits. In the block exposure process, a block mask (transparent mask) arranged at the center of an optical axis of the exposure system is used. The block mask has a plurality of slits having respective shapes corresponding to unit block patterns (hereinafter, the term "unit" will sometimes be omitted for the sake of simplicity). One of the slits of the block mask is selected on the basis of a pattern to be drawn on a wafer, and an electron beam is projected onto the selected slit of the block mask. The electron beam passes through the selected slit, and is projected onto the wafer. The pattern actually formed on the wafer is a reduced version of the selected slit.
Block exposure data used in the block exposure process is generated in the following manner. Parts (unit blocks) which can be exposed in the block unit are manually extracted from a pattern shown in the design drawing. Then, the design drawing is manually rearranged taking into account the extracted parts of the pattern. As the number of block patterns (that is, the number of slits having different shapes) increases, it becomes more difficult to manually extract parts corresponding to the block patterns and manually rearrange the original design drawing. The above extracting and rearranging procedures may be performed by means of a computer. However, a very complicated sequence will be needed to perform the above extracting and rearranging procedures.
A further description will now be given of the conventional extracting and rearranging procedure. FIG. 1 is a block diagram of a system for performing the conventional extracting and rearranging procedure, and FIG. 2 is a diagram showing how a unit block corresponding to one silt of the mask is extracted from a pattern.
The system shown in FIG. 1 is made up of a computer system 11. a keyboard 12, and a display unit 13. An operator inputs information D to be operated contained in an exposure pattern (exposure data) to the computer system 11 by means of the keyboard 12. The computer system 11 calculates a predetermined operation on the input exposure data, and obtains coordinates values of apexes of the exposure pattern, which coordinate values are then displayed on the display unit 13.
FIG. 2 shows an exposure pattern PIN. The exposure data of the exposure pattern PIN includes coordinates of an exposure reference point defined as an origin O(x0, y0), and coordinates of apexes of exposure pattern PIN with respect to the origin O, (x1, y1), (x2, y2), . . . , (x32, y32). Further, the exposure data includes vector information concerning vectors connecting the adjacent apexes to each other. Hence, the exposure data indicates the contour of the exposure pattern PIN.
FIG. 3 shows a unit block exposure pattern to be extracted from the exposure pattern PIN shown in FIG. 2. The operator sees the exposure pattern PIN displayed on the display unit 13, and manually extracts the unit block exposure pattern from the exposure pattern PIN. In this case, the operator selects predetermined unit block exposure patterns one by one, and compares the selected unit block pattern with the exposure pattern PIN. When it is determined that the block pattern shown in FIG. 3 can be extracted from the exposure pattern PIN, as shown in FIG. 2, exposure data indicating a part of the exposure pattern PIN corresponding to the block pattern is replaced by data indicating the identified block pattern. In this case, the coordinate data shown in FIG. 4 to be replaced by the block pattern is specified so that the corresponding part of the pattern PIN is segmented as a block pattern. In the above-mentioned manner, the exposure data input to the computer system 11 is completely accessed and it is determined whether or not the exposure data can be replaced by the block exposure data.
However, the above-mentioned conventional extracting and rearranging method and system have the following disadvantages. First, it is necessary to manually make a decision as to whether or not block patterns can be extracted from exposure data originally designed for the variable rectangle-shaped beam exposure system. As the integration density of LSI circuits increases, it becomes more difficult to manually make the above decision and the time necessary to complete the extracting and rearranging operation greatly increases. Second, when there is no extractable block pattern, it is necessary to compare all the block patterns with the exposure pattern within a specified range, one by one, until the block patterns cannot be extracted at all. The time necessary for the above comparing operation is wasteful.